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In the year 1905, a young Albert Einstein published a number of

scientific papers that changed physics forever. The best known of these,

the Special Theory of Relativity, quickly established the young Einstein as

a scientist of note (see J.IEI vol. 59:6) and led to Einsteins General Theory

of Relativity, one of the pillars of modern physics. In a second paper,

Einstein published a controversial proposal concerning the nature of light

that later formed a cornerstone of quantum theory, the revolutionary

theory that underpins much of modern science and technology (see J.IEI

vol. 59:7). Incredibly, the young Einstein made a third ground-breaking

advance in 1905.

He published an analysis that pointed the way towards a crucial test of

the reality of atoms, and of the validity of the laws of thermodynamics.

The outcome of that test now underpins much of modern science,

from our view of the atomic nature of matter to our understanding of

meteorology and other complex systems.

Atoms and chemistryThe idea that all matter is made up of minute, indivisible entities called

atoms was first put forward by the philosophers of ancient Greece. The

concept gained much credibility in the 19th century when scientists such

as John Dalton used it to establish laws of chemistry that successfully

described how the chemical elements combine to form molecules.

A listing of the known elements in order of increasing atomic

weight led to the development of The Periodic Table by Mendeleyev, a

development that revolutionized the study of chemistry. It was widely

assumed that the properties of a given element was determined by

the properties of its constituent atoms - however, there was no direct

evidence of the existence of atoms, and some eminent scientists did not

believe in the atomic hypothesis.

Enter EinsteinGreatly interested in the atomic view of matter, the young Einstein

devised a mathematical method of calculating the size of atoms and

molecules in early 1905. From an analysis of sugar molecules dissolved

in water, he calculated both the diameter of the sugar molecule and

Avogadros number (the number of molecules per unit volume under

standard conditions) from the known viscosity of the liquid and the

diffusion rate of sugar.

His calculations were in good agreement with previous theoretical

estimates and were well-received. However, the very existence of atoms

and molecules had still to be demonstrated in convincing fashion, and

the 26-year-old Einstein applied himself to this task.

Today, Einsteins notion of statistical fluctuations has found application throughout the sciences.

Einstein and statistics According to the atomic view of matter, a liquid is made up of a huge

number of molecules in random, ceaseless motion, the properties of the

liquid arising from the average behaviour of its constituent molecules.

Working from first principles, Einstein made a careful study of the

statistics of such an assembly and, in May 1905, he made a key proposal

concerning its behaviour.

He proposed that any such system would experience statistical

fluctuations, during which random elements depart from their average

behaviour (just as a dice player can occasionally throw several sixes in

a row).

Applying this concept of statistical fluctuation to the case of molecules

in liquids, Einstein proposed that a small group of neighbouring

Einstein and the Atomic Theory

Albert Einstein : another 1905 paper facilitated a test for atomic theory

In the third of a series of articles celebrating Einsteins Miraculous Year, Cormac ORaifeartaigh describes Einsteins ground-breaking contribution to atomic theory.

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molecules could momentarily move in the same direction a fluctuation

that would cause a body immersed in the liquid to experience a tiny

push in that direction. Another group of molecules could cause the same

body to experience a tiny push in a different direction moments later and

the immersed body would therefore experience a zig-zag motion in the

liquid a motion that might be observable. Hence, while the molecules

of a liquid were far too small to be observed directly by microscope, their

motion might be detectable by its effect on a larger particle suspended

in the liquid!

Brownian MotionExcitingly, a zig-zag motion of particles suspended in a liquid had long

been known to scientists (named Brownian Motion after the English

botanist who studied the effect in detail). The cause of this motion had

been a great mystery and accurate measurement of the 3-D random

motion of an immersed particle had proved extremely difficult. Here,

Einstein made a second vital contribution.

Starting with the assumption that the motion was indeed due to a

buffeting of the immersed particle by the molecules of the liquid, he

calculated the average horizontal distance an immersed particle would

travel in a given time. Hence, from his own statistical analysis, Einstein

delivered a well-defined, measurable estimate that could be easily tested.

The experimentThe French scientist Jean Perrin rose to Einsteins challenge with a series

of experiments in 1908. Equipped with nothing but a microscope and

a stopwatch, Perrin and his team measured the horizontal displacement

of gum extract particles suspended in water as a function of time. The

data were in exact accord with Einsteins predictions, giving the world

the first unequivocal evidence of the reality of molecules. Einstein was

delighted, as was Perrin the Frenchman was later awarded the Nobel

Prize for this work!

Young Einstein devised a mathematical method of calculating the size of atoms and molecules in early 1905.

ImplicationsEinsteins Brownian-motion paper facilitated the first real glimpse

of the atomic nature of matter, an advance that underpins almost all

of modern science. Another consequence of the paper was that, since

the properties of matter were now known to be determined by the

behaviour of huge numbers of atoms, it was realized that the laws of

thermodynamics were valid only in a statistical sense.

For the first time, the role of probability in the laws of physics was

established, a defining moment in the philosophy of science.

Modern ApplicationsToday, Einsteins notion of statistical fluctuations has found application

throughout the sciences. From the study of cell membranes to our view

of evolution, from the analysis of weather systems to the study of the

stock market, it underpins our understanding of all complex systems.

Perhaps the Brownianmotion paper did not have quite the dramatic

impact of the Special Theory of Relativity, or indeed of Einsteins

quantum view of light but it resulted in a quiet revolution that has had

an lasting influence on modern science.

Dr Cormac ORaifeartaigh lectures in physics at Waterford Institute

of Technology

Above: Jean Perrin was awarded the 1926 Nobel Prize for his experimental verification of Einsteins predictions.

Left: The notion of statistical fluctuations underpins our view of complex systems such as the weather.

Brownian motion : the random motion of a particle suspended in a liquid was first studied by botanist Robert Brown (1773-1858).

Credit: NASA.

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